Differential Diagnosis of Metabolic Acidosis

Occult hypoperfusion (OH) entails inadequate tissue oxygenation in the presence of normal vital signs. Numerous studies have demonstrated that this phenomenon is associated with impaired outcome and increased mortality, however definitions of OH differ between studies. The aim of the current study was to identify and evaluate the published definitions of the term `occult hypoperfusion` in trauma (tOH).

A review of literature was performed using the Medline database. The following MeSH terms have been used: occult hypoperfusion, severe trauma, polytrauma, resuscitation. Clinical studies on OH were included and utilized definitions have been compared. A predefined data-interpretation process was applied to create an integrative definition for tOH.

A total of 43 publications used the term OH. A definition of OH was provided in 16 manuscripts. A pooled mean number of participants of 729 (SD: 1158) was found per study. The majority of manuscripts combine multiple parameters for inadequate tissue oxygenation and normal vital signs to define OH in trauma. In 12 manuscripts, specific cut-off values for hemodynamic parameters (SBP/PR/UP) and in 11/16 papers exact metabolic thresholds (lactate/B.E) were defined. The following definition best integrates definitions in current literature: Lactate > 2 mmol/l or BE <-3 mmol/l (to define inadequate oxygenation) plus SBP > 90 mmHg and PR < 120 bpm (to define normal macroperfusion).

The current systematic review demonstrates that definitions of occult hypoperfusion in trauma differ in the literature. The following comprehensive definition for (tOH) is proposed: Lactate > 2 mmol/l or BE <-3 mmol/l plus SBP > 90 mmHg and PR < 120 bpm. This recommendation appears to represent current literature on tOH and may improve the identification of trauma patients at risk for OH and related complicated courses. Further validation studies are required to demonstrate the clinical role of tOH and the proposed definition.

Overdoses are associated with significant morbidity and mortality. Evaluation and treatment of the overdose patient should be systematic and based on excellent supportive care. Attention to the airway, breathing, and circulation status of the patient is paramount. While frequently difficult to obtain, appropriate history regarding the exposure can be crucial and should focus on what, when, and how much. The physical exam should be thorough and focus on signs consistent with toxidromes or other findings that may help determine what the patient overdosed on. Laboratory evaluation can be proved valuable when history and physical exam are limited but should always be correlated with the clinical exam. Symptomatic and supportive care is the initial best treatment for any overdose patient. Decontamination should be considered but not routinely used. Antidotal therapy can be lifesaving in specific exposures. Asymptomatic overdose patients should be observed for at least 6 hours and those that develop symptoms should be admitted to the further treatment and observation.

Equine asthma syndrome is an allergic, inflammatory airway disease that usually affects older horses. Respiratory acidosis is an acid–base imbalance caused by alveolar hypoventilation. The acid–base balance may be assessed using the Henderson–Hasselbalch equation as well as the Stewart model. The authors hypothesized that systemic respiratory acidosis changes the ionic concentrations affecting water dissociation. The study group included 16 Warmblood, mixed breed horses of both sexes with a history of severe equine asthma, and 10 healthy horses were used as controls. Arterial and venous blood were collected from all the horses. The pH, pO₂, and pCO₂ and HCO₃₋ were assessed in the arterial blood. Na, K, Cl, albumin, and P_inorganic (P_i) were assessed in the venous blood. The obtained results were used to calculate the anion gap (AG), modified AG, actual strong ion difference (SID_a), weak non-volatile acids, and effective strong ion difference (SID_e) values for all the horses. A systemic, compensatory respiratory acidosis was diagnosed in the study group. The concentration of Na in the blood serum in the study group was significantly higher, whereas the concentration of Cl was significantly lower than the values in the control group. The SID_a and SID_e values calculated in the horses from the study group were significantly higher than those in the control group. Significantly higher SID_a and SID_e values confirm the presence of ionic changes that affect water dissociation in the course of respiratory acidosis in horses. The SID_a and SID_e values may be useful in the diagnosis and treatment of respiratory acidosis in horses, which warrant further investigation.

Sodium thiosulfate (STS) is used to treat calciphylaxis and cyanide poisoning, but can lead to a serious anion-gap acidosis. We suspected that the calculated anion gap in a patient treated with STS for calciphylaxis was decreased to normal by a falsely increased chloride, and we hypothesized that STS directly interfered with chloride measurements.

Plasma pools were prepared with 12 concentrations of STS from 0 to 20 mmol/l. Chloride was measured in each sample on 9 analyzers: Architect 16200, StatProfile pHOx Plus, RapidLab 1265®, Vitros 350®, Advia 1800, Roche Modular, iSTAT1, RAPIDpoint 500, and Radiometer ABL735.

Statistically significant, dose-dependent increases in reported chloride concentrations were seen with all analyzers except the RAPIDpoint 500 and Vitros. The increases ranged from 5 to 75 mmol/l at the peak thiosulfate concentrations (33 mmol/l) expected in treated patients. The CLIA-allowable error of 5% was exceeded by 4 analyzers (Architect 16200, iSTAT1, StatProfile pHOx Plus, and Radiometer ABL735). The RAPIDpoint 500 showed a 3-mmol/l decrease in measured chloride over the tested range. The Vitros analyzer showed no interference.

Interference of STS in chloride measurement in several common analyzers may lead to erroneous anion-gap calculations and confound the diagnosis of STS-induced anion-gap acidosis.